As tempting as it would be to have the complication of "oh noes, Sarah can't reach the scales!", never give the characters a problem you yourself don't have a solution to.
Because, seriously, that's like, the top of a stone building from Sarah's perspective! Her only hope would be too oh my god I just now figured out how she could do it. She could go back to the first puzzle and intentionally become a lizard again.
Granted, I could just claim that puzzle was already solved and that transformation doesn't work at the moment, but I was genuinely writing this and was like "I can't think of a single reasonable way she could wait there it is it's right there if she can still turn part lizard that would totally 100% work".
More I think on it, though, that transformation wouldn't work right now, or it would reset the puzzle, or mess up the balancing, or... Yeah, okay, it's probably not a good idea, but if I DID want to have "I can't reach the pedestal" as a complication, that could've been a potential solution.
Figuring out how much to grow Nanase by relative to Sarah's shrinking was... Well, the answer's not complicated, and the actual math simple, but I had to get to the point of knowing what math to do, and my morning brain was not immediately up to the task.
(Before I proceed, there is rounding to the nearest hundredth afoot, so several of these numbers are approximate.)
Ultimately, I just wanted Nanase to gain in size however much Sarah lost. Sarah wound up at 1/4 scale, which is actually 1.56% of her normal size (0.25 cubed). This means she lost about 98.44% of her size, which I decided meant Nanase should become 198.44% of HER normal size, so about 125.66% size (cube root of 1.98).
In other words, even though Sarah's at 1/4 scale, Nanase winds up at nearly 200% her normal size, which is only about 1.25/1 scale. This is why big lads like elephants don't run around doing back flips, folks.
At one point, however, I was looking at things the wrong way, and Nanase nearly wound up about 3:1 scale. Somehow. I can't even remember how I got to that point.
Also, apologies to everyone who would totally be rooting for 2,700% size Nanase.
Incidentally, as much as I'm a fellow who enjoys odd transformation nonsense like two people winding up identical by averaging out their physical characteristics, THIS is the actual reason that's part of this story. This entire sequence would be way more difficult to do if Sarah and Nanase's base forms for it weren't identical. I'm not even entirely sure how I'd do it.
...Maybe an approximate adjustment based on height with an estimate for additional STOP SUDDENLY FIGURING STUFF OUT, BRAIN